1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus, and more particularly to a discharge lamp lighting apparatus adapted to light multiple discharge lamps for use as a backlight in a liquid crystal display apparatus.
2. Description of the Related Art
A liquid crystal display (LCD) apparatus is extensively used in electronics devices, and is increasingly superseding a cathode ray tube (CRT) in a relatively large-screen display apparatus such as a personal computer, and a TV set. In such a large-screen LCD apparatus, a backlight system provided with a plurality of discharge lamps is generally used in order to ensure a sufficient display brightness and also uniformity in the brightness, and the discharge lamps are usually driven by a discharge lamp lighting apparatus having an inverter circuit.
Such an LCD apparatus advantageously has a thinner profile and lower power consumption than a CRT conventionally used, but incurs a deteriorated image quality due to a blurred outline in a movie display. Specifically, in the LCD apparatus which is a hold-type display apparatus, the brightness of each pixel is held substantially constant throughout a frame period until an image data of a subsequent frame period is displayed. Consequently, when the outline of a moving object is followed, an observer recognizes an image produced such that gradation values at surrounding areas of respective outlines over several frame periods are synthesized, and it looks to the observer that the outline is blurred. On the other hand, in the CRT which is an impulse-type display, the brightness of each pixel is attenuated in a frame period, and therefore the observer is kept from recognizing gradation values at surround areas of respective outlines as synthesized at the transition time from one frame period to the next frame period, thus keeping off the blurred outline problem described above.
In addressing the problem, a backlight is blinked during one frame period thereby realizing an impulse-type light emission in an LCD apparatus (refer to, for example, Japanese Patent Application Laid-Open No. 2002-110393). Specifically, a plurality of discharge lamps constituting a backlight are arranged parallel to scanning lines of a screen, and some thereof corresponding to a partial region of the screen are turned on in a frame period and then turned off after a predetermined time in the same frame period thus performing a blinking operation. The blinking operation is sequentially repeated in synchronization with the scanning lines to sequentially cover a plurality of partial regions (such a operation is referred to as “blinking lighting”), whereby the problem of a blurred outline is reduced or eliminated. In order to perform such a blinking lighting, a discharge lamp lighting apparatus must be capable of controllably lighting on and off the plurality of discharge lamps individually.
FIG. 3 is a circuitry of a conventional discharge lamp lighting apparatus 100 employing inverters to perform a blinking lighting. The discharge lamp lighting apparatus 100 includes step-up transformers T1 to Tn, discharge lamps La1 to Lan connected respectively to the secondary sides of the step-up transformer T1 to Tn, and inverters INV1 to INVn connected respectively to the primary sides of the step-up transformer T1 to Tn and adapted to light the discharge lamps La1 to Lan. The inverters INV1 to INVn are separate oscillating inverters each including a control circuit 101 with a control IC, and a bridge circuit 102. The control circuit 101 drives the bridge circuit 102 thereby generating an AC voltage at the primary side of each of the step-up transformers T1 to Tn, then a high voltage is induced at the secondary side of each of the step-up transformers T1 to Tn, and the discharge lamps La1 and Lan are lighted by respective high voltages induced. Lamp current detecting resistors R1 to Rn are provided respectively at the secondary sides of the step-up transformers T1 to Tn, and voltage values thereof are fed back to the control circuit 101 via respective diodes D1 to Dn, whereby lamp currents flowing through respective discharge lamps La1 to Lan are maintained at a prescribed value. Control IC's of respective control circuits 101 have input terminals for synchronizing signals P1 to Pn and cause respective bridge circuits 102 to start and stop their operations according to the synchronizing signals P1 to Pn, whereby the discharge lamps La1 to Lan are individually lighted on and off.
FIG. 4 is a circuitry of another conventional discharge lamp lighting apparatus 200 with a blinking lighting operation. The discharge lamp lighting apparatus 200 includes a DC-DC converter 201, a plurality of discharge lamps La1 to Lan, and a plurality of inverters INV1 to INVn. The inverters INV1 to INVn are self oscillating inverters each including a corrector resonance Royer circuit 202 including a step-up transformer T1, and its operating voltage is controlled and maintained by the DC-DC converter 201. In the Royer circuit 202, a bias voltage is supplied to switching elements Q7 and Q8 via a switching element Q9, and the Royer circuit 202 is caused to start and to stop its operation by turning on and off the switching element Q9 according to a synchronizing signal P1/P2/ . . . /Pn, whereby the discharge lamp La1/La2/ . . . /Lan is lighted on and off.
The discharge lamp lighting apparatus 100 shown in FIG. 3 requires a plurality of control IC's in order to light on and off the plurality of discharge lamps, thus increasing the cost of the apparatus. The discharge lamp lighting apparatus 200 shown in FIG. 4, which does not requires a control IC and therefore is advantageous in cost compared to the apparatus 100, incurs a large loss due to heat generated, and the like, thus raising a problem in efficiency.